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| United States Patent |
5,780,823
|
|
Uehashi
|
July 14, 1998
|
Cooking method using a microwave oven
Abstract
Since, food sealed tightly in a packaged does not permit steam to be
released from the food during heating, conventional microwave ovens using
a hygrometer cannot identify the type of food, and an automatic heating is
therefore impossible. A microwave oven of the present invention includes:
a temperature detector for detecting the temperature at the top of the lid
of the package of a packaged food; a first calculating unit for
calculating a first parameter representing a degree of rise in the
detected temperature during a period of time since the heating is started
until a predetermined change in the detected temperature is attained; a
second calculating unit for calculating a second parameter representing a
degree of rise in the temperature after the predetermined change in the
temperature is attained. A controller judges the amount of food in the
packaged food based on the first parameter, and judges the packing state
of the packaged food based on the second parameter. Based on the
judgements, or according to the parameters, the controller selects an
appropriate heating mode for the packaged food out of a plurality of
heating modes varying in the maximum heating time and in the maximum
temperature. The heating unit (magnetron) is controlled by the controller
according to the heating mode selected.
| Inventors:
|
Uehashi; Hiroyuki (Koka-gun, JP)
|
| Assignee:
|
Sanyo Electric Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
804539 |
| Filed:
|
February 24, 1997 |
Foreign Application Priority Data
| Current U.S. Class: |
219/710; 99/325; 219/711; 219/719; 219/734 |
| Intern'l Class: |
H05B 006/68 |
| Field of Search: |
219/710,711,705,734,719
99/325
426/243
|
References Cited
U.S. Patent Documents
| 4281022 | Jul., 1981 | Buck.
| |
| 4401884 | Aug., 1983 | Kusunoki et al.
| |
| 4831227 | May., 1989 | Eke | 219/710.
|
| 4894502 | Jan., 1990 | Oh | 219/710.
|
| 5422465 | Jun., 1995 | Kim et al. | 219/710.
|
| 5530229 | Jun., 1996 | Gong et al.
| |
| Foreign Patent Documents |
| 0-587-323-A1 | Mar., 1994 | EP.
| |
| 2-46101 | Jul., 1984 | JP.
| |
| 64-5435 | Oct., 1984 | JP.
| |
| 63-207921 | Aug., 1988 | JP | 219/711.
|
| 63-201429 | Aug., 1988 | JP | 219/711.
|
| 4-90420 | Mar., 1992 | JP.
| |
| 2-206-425 | Jan., 1989 | GB.
| |
Primary Examiner: Leung; Philip H.
Attorney, Agent or Firm: Oliff & Berridge
Claims
What is claimed is:
1. A cooking method using a microwave oven having means for heating by
microwave an object contained in a container with a lid and means for
detecting a temperature at a top of the lid of the container, the cooking
method comprising the steps of:
calculating a first parameter representing a degree of rise in the
temperature detected by the temperature detecting means in an initial
phase of a heating of the object;
calculating a second parameter representing a degree of rise in the
temperature detected by the temperature detecting means after calculating
the first parameter; and controlling the heating means based on the first
and second parameters.
2. The method according to claim 1, wherein said first parameter is
calculated during a predetermined period of time in the initial phase of
the heating of the object; and the second parameter is calculated after
the predetermined period of time.
3. The method according to claim 2, wherein the first parameter represents
the degree of rise in the temperature during a period of time from when
the heating of the object is started until a predetermined change in the
temperature detected by the temperature detecting means is attained.
4. The method according to claim 3, wherein at least one of the step of
calculating the first parameter or the step of calculating the second
parameter further includes the step of calculating a time required for the
temperature to rise by a predetermined amount.
5. The method according to claim 3, wherein at least one of the step of
calculating the first parameter or the step of calculating the second
parameter further includes the step of calculating a change in the
temperature per unit time.
6. The method according to claim 3, wherein the step of calculating the
first parameter includes calculating a period of time since the heating is
started until a predetermined change in the temperature is attained, the
step of calculating the second parameter includes calculating a change in
the temperature per unit time since the predetermined change in the
temperature is attained, and the step of controlling the heating means
includes controlling the heating means based on the period of time and the
change in the temperature per unit time.
7. The method according to claim 3, wherein the step of controlling the
heating means includes controlling the heating means by changing a maximum
heating time and/or a maximum temperature according to the first and
second parameters.
8. The method according to claim 2, wherein the first parameter represents
the degree of rise in the temperature during a period of time from when
the heating of the object is started until a predetermined value of the
temperature detected by the temperature detecting means is attained.
9. The method according to claim 8, wherein at least one of the step of
calculating the first parameter or the step of calculating the second
parameter further includes the step of calculating a time required for the
temperature to rise by a predetermined amount.
10. The method according to claim 8, wherein at least one of the step of
calculating the first parameter or the step of calculating the second
parameter further includes the step of calculating a change in the
temperature per unit time.
11. The method according to claim 8, wherein the step of controlling the
heating means includes controlling the heating means by changing a maximum
heating time and/or a maximum temperature according to the first and
second parameters.
12. The method according to claim 2, wherein at least one of the step of
calculating the first parameter or the step of calculating the second
parameter further includes the step of calculating a time required for the
temperature to rise by a predetermined amount.
13. The method according to claim 2, wherein at least one of the step of
calculating the first parameter or the step of calculating the second
parameter further includes the step of calculating a change in the
temperature per unit time.
14. The method according to claim 2, wherein the step of controlling the
heating means includes controlling the heating means by changing a maximum
heating time and/or a maximum temperature according to the first and
second parameters.
Description
The present invention relates to a microwave oven, particularly to a heat
control system of the microwave oven suitable for heating a packaged food
seated up with a lid or by a plastic wrap.
BACKGROUND OF THE INVENTION
Some conventional microwave ovens are known that can automatically detect
the type of food put in the oven and determine an appropriate heating
time. One of these such microwave ovens is disclosed in the Japanese
Examined Patent Publication No. H2-46101 (which corresponds to the
Japanese Unexamined Patent Publication No. S59-120949). The microwave oven
includes a humidity sensor, or hygrosensor, disposed in the heating
chamber. The hygrosensor measures the absolute humidity in the heating
chamber due to the steam released from the food heated. Based on the
change in the humidity per unit time, the type of the food is identified,
and the amount of input heat and the heating time are determined according
to the type identified.
In convenience stores or food shops, food is often sold packaged in a tray
with a lid or in a tray covered by a plastic wrap made of vinylidene
chloride or the like. The tray and the lid are sometimes tightly sealed
and the plastic wrap sometimes wraps the whole package hermetically.
When a packaged food is heated, steam is given off from the food. If,
however, the package is tightly sealed, the steam does not leak out but is
kept inside the package. In such case, in the above described conventional
microwave oven, the type of food is wrongly identified, and the amount of
input heat and the heating time thus determined are inappropriate for the
food. This tends to lead to an overheating wherein the packaged food is
heated for a considerably long time. If the package is made of plastic or
the like, it may be deformed by heat and the steam leaks out of the
package and the wrapping, which may be detected by the hygrosensor too
late.
The Japanese Examined Patent Publication No. S64-5435 (which corresponds to
Japanese Unexamined Patent Publication No. S59-175588) discloses another
microwave oven constituted so that the temperature at the surface of the
food is detected by an infrared sensor. The size of the food is identified
based on the rising rate of the temperature detected during the heating
process, and an appropriate heating time is determined according to the
size identified. When, however, a packaged food with a transparent lid is
heated, the temperature detected by the infrared sensor mainly reflects
the temperature at the surface of the lid or the plastic wrap, not the
temperature at the surface of the food, because the infrared detection can
hardly pass through the lid. Further, the manner in which the temperature
rises depends not only on the type or size of the food in the package, but
considerably on whether the food is in contact with the lid. Thus, in the
conventional microwave oven, the size of the food is sometimes wrongly
identified and the food is heated inappropriately in the case of packaged
foods.
The Japanese Unexamined Patent Publication No. H4-90420 discloses a
microwave oven particularly suitable for cooking rice. The microwave oven
includes a thermosensor for measuring the temperature at the lid of the
package containing rice and water, and a hygrosensor for measuring the
absolute humidity due to the steam released from the food, both being
disposed in the heating chamber. In an initial phase of the heating, the
type of the lid is identified based on a change in the temperature in a
predetermined period of time. Then the amount of rice is detected based on
a change in the temperature per unit time and on a time required for the
absolute humidity to rise by a predetermined amount.
The above microwave oven, however, can be applicable only to a particular
sort of cooking, i.e. rice cooking, and cannot be suitably applied to
heating foods of various package sizes and various food types. Further,
the microwave oven is not suitable for heating a sealed packaged food
because, as described above, the humidity detection is meaningless in such
a case.
DISCLOSURE OF THE INVENTION
The present invention is accomplished in view of the above described
problems, and an object of the present invention is to provide a microwave
oven that can preferably heat and cook a sealed packaged food. A microwave
oven according to the present invention includes:
a) heating means for heating by microwave an object contained in a
container with a lid;
b) temperature detecting means for detecting the temperature at the top of
the lid of the container;
c) first calculating means for calculating a first parameter representing a
degree of rise in the temperature during a predetermined period of time in
an initial phase of a heating of the object;
d) second calculating means for calculating a second parameter representing
a degree of rise in the temperature after the predetermined period of
time; and
e) control means for controlling the heating means based on the first and
second parameters.
The lid cited above includes a plastic wrap that wraps the container
containing the object.
In heating an object in a container, or, more specifically, a packaged
food, how long it takes for the temperature at the lid to rise by a
predetermined amount depends on the amount of food contained therein.
After the temperature rises by the predetermined amount, however, the
change in the temperature per unit time depends on the type of the food
and on the size of the space between the food and the lid. The reason is
explained as follows. When the food is heated, hot steam or gas is
released from the food, which fills the space between the food and the
lid, so that the temperature of the lid rises due to the heat conducted
from the steam or gas to the lid. The heat capacity of the lid made of
vinyl chloride or the like is normally smaller than that of the food.
Therefore, if the space between the food and the lid is large, the
temperature at the lid is largely influenced by the hot steam or gas,
resulting in a larger change in the temperature per unit time. If, on the
other hand, the food is in contact with the lid, the temperature of the
lid is almost equal to that of the food, resulting in a smaller change in
the temperature per unit time.
In the microwave oven according the present invention, the first
calculating unit calculates the first parameter representing the degree of
rise in the temperature in an initial phase of the heating, and the second
calculating unit calculates the second parameter representing the degree
of rise in the temperature in a phase following to the initial phase. The
controller judges the amount of food in the packaged food based on the
first parameter, and judges the packing state of the packaged food based
on the second parameter, as explained above. Based on the judgements, or
according to the parameters, the controller selects an appropriate heating
mode for the packaged food out of a plurality of heating modes varying in
the maximum heating time and in the maximum temperature. The heating unit
(e.g. a magnetron) is controlled by the controller according to the
heating mode selected.
In the microwave oven according to the present invention, the first
calculating unit may calculate the first parameter representing the degree
of rise in the temperature during a period of time since the heating of
the object is started until a predetermined change in the temperature
detected by the temperature detecting unit is attained. The first
calculating unit may otherwise calculate the first parameter representing
the degree of rise in the temperature during a period of time since the
heating of the object is started until a predetermined value of the
temperature detected by the temperature detecting unit is attained.
In the microwave oven according to the present invention, the first and/or
second calculating unit may be constituted so that the time required for
the temperature to change by a predetermined amount is utilized as the
parameter representing the degree of rise in the temperature. The first
and/or second calculating unit may be otherwise constituted so that the
change in the temperature per unit time is utilized as the parameter
representing the degree of rise in the temperature.
For example, the microwave oven according to the present invention may be
constituted so that the first calculating unit calculates the time
required for the temperature to change by the predetermined amount since
the start of heating, whereafter the second calculating unit calculates
the change in the temperature per unit time, and the controller controls
the heating unit according to the above time required and the above change
in the temperature per unit time.
In addition, in the case where the first calculating unit utilizes the time
required for the temperature to change by the predetermined amount since
the start of heating as the parameter representing the degree of rise in
the temperature in the initial phase of heating, it is necessary to
measure the above time while the amount of the steam or gas released from
the food is small. Accordingly, for example, it is preferable that the
above predetermined amount is set at about 15-20 ›.degree.C.!.
By the microwave oven according to the present invention, various packaged
foods can be cooked properly by the heating mode automatically selected
according to the type, amount and packing state of the food packaged in
the container. Therefore, the user need not judge the type or amount of
food and it is no longer necessary to set the heating temperature, heating
time, etc. by himself or by herself, but the packaged food can be heated
safely without resulting in excessive heating or insufficient heating.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiment of the present invention will be described in the
following part of the specification referring to the attached drawings
wherein:
FIG. 1 is a vertical cross-sectional view of the microwave oven in the
embodiment of the present invention;
FIG. 2 is a schematic block diagram showing the electrical system of the
microwave oven according to the embodiment;
FIG. 3 is a flow chart showing control steps of the heating in the
embodiment;
FIG. 4 shows an example of the heating sequence; and
FIG. 5 is a graph for explaining that the change in the temperature depends
on the food.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a vertical cross-sectional view of a microwave oven embodying the
present invention. In a casing 1 of the microwave oven, a heating chamber
2 is disposed, and a magnetron 3 for generating microwave is provided at
the back of the heating chamber 2 via a waveguide 4. A turntable 6 rotated
by a turntable motor 7 is provided at the bottom of the heating chamber 2,
and a heating object 5 is placed on the turntable 6. On the top of the
heating chamber 2 is disposed an infrared sensor 8, whereby it is able to
detect and measure the temperature emitted by infrared signals from the
top of the object 5. An exhaust duct 9 is provided at the top of the
heating chamber 2 for letting air or steam out of the heating chamber 2,
and a hygrosensor 10 is provided in the exhaust duct 9. In this
embodiment, the object 5 consists of a food 5d packaged in a tray 5b with
a lid 5a and a plastic wrap 5c wrapping the tray 5b and the lid 5a.
FIG. 2 is a block diagram showing the electrical system of the above
microwave oven. A controller 20 is composed of one or plural
microcomputers and peripheral elements including a timer 21, random access
memory (RAM) 22, etc. An operation unit 23 having a plurality of keys is
connected to the controller 20, and signals corresponding to operations on
the keys are sent to the controller 20. To the controller 20, a
temperature signal and a humidity signal are also sent from the infrared
sensor 8 and the hygrosensor 10, respectively. Responsive to the input
signals, the controller 20 executes calculations according to control
programs stored in a read only memory (ROM) (not shown), and generates
various control signals. The control signals include a signal for driving
the magnetron 3, a signal for controlling a lamp 24 disposed in the
heating chamber 2, a signal for driving the turntable motor 7, and a
signal for driving a blower motor 25 for an exhaust fan disposed in the
exhaust duct 9 (not shown).
Next, referring to the flow chart in FIG. 3, a heating by the microwave
oven constituted as above is described. First a user places a packaged
food as the object 5 on the turntable 6, selects an "Automatic Heating for
Packaged food" course on the operation unit 23, and pushes a start key
(step S1). In response to the key operation, the controller 20 turns on
the lamp 24 and sends start signals to the turntable motor 7, the blower
motor 25 and the magnetron 3 (step S2), whereby the turntable 6 starts
rotating at a low speed and the irradiation of microwave is started. The
power of the magnetron 3 is determined by a heating sequence selected as
explained later. At the moment the heating is started, the timer 21 in the
controller 20 starts counting time (step S3).
Since, generally, it takes a certain short period of time for the infrared
sensor 8 to be stabilized after it is energized, the operation is
suspended and the temperature signal is disregarded for the first 5 ›sec!
(step S4). When the timer 21 counts up 5 ›sec!, the controller 20 starts
using the temperature signal from the infrared sensor 8 (step S5). The
temperature obtained based on the temperature signal from the infrared
sensor 8 is hereinafter referred to as the "detected temperature". In the
above process, first the detected temperature obtained just after the
process is started is stored in the RAM 22 as T0 (step S6). When the
detected temperature attains T0+18 ›.degree.C.! (step S7), a time length
t0 required for the temperature to rise by 18 ›.degree.C.! is calculated
by subtracting 5 ›sec! (which is the time elapsed in step S4) from the
current time counted by the timer 21. The time length t0 calculated here
and the current temperature T1 ›.degree.C.! (=T0+18 ›.degree.C.!) is
stored in the RAM 22 (steps S8, S9).
From the time when the detected temperature attains T0+18 ›.degree.C.!, the
operation is again suspended for 5 ›sec! (step S10). When the 5 ›sec!
elapses, the current temperature T2 ›.degree.C.! is detected and the
temperature change .alpha. ›.degree.C.! in the 5 ›sec! is calculated as
.alpha.=T2-T1 (step S11), where T1 is restored from the RAM 22. The
temperature change .alpha. is stored in the RAM 22 (step S12). Based on
the time length t0 and the temperature change .alpha., a heating mode is
selected from a plurality of the heating modes stored in the ROM
beforehand (steps S13, S14).
FIG. 4 shows an example of a list of heating modes classified by the
parameters of the time length t0 and the temperature change .alpha.. Each
heating mode provides a heating sequence including a first stage, second
stage and third stage. In FIG. 4: "t" used in the third stage represents
the length of time from the beginning of the first stage to the end of the
second stage; "tmax" is the maximum heating time; and "Tmax" is the
maximum temperature. When the maximum temperature Tmax is attained, or
when the maximum heating time tmax elapses, whichever occurs first, the
operation proceeds to the third stage in case of the second stage, or the
operation is finished in case of the third stage. The heating sequences
are classified into three patterns A, B and C according to the time length
t0. Each of the three patterns A, B and C further includes three modes
corresponding to the temperature change .alpha.. Thus, one of the nine
modes of A1-A3, B1-B3 and C1--C3 is selected according to the time t0 and
the temperature change .alpha..
The heating sequences as shown in FIG. 4 are normally predetermined through
prior experiments in which a variety of packaged foods are actually
heated. The heating sequences are stored as a part of the control programs
in the ROM provided in the controller 20.
FIG. 5 shows a result of experiments wherein three samples of the packaged
foods were heated. In FIG. 5, the abscissa represents the time elapsed
since the heating is started, the ordinate represents the detected
temperature, and the legends A3, B2 and C1 beside the curves correspond to
modes in FIG. 4.
In Japan, various packaged foods are sold in convenience stores, food
shops, etc. Some of the examples are as follows. "Souzai" is one of a
variety of foods, such as, for example, boiled beans, shredded vegetables,
etc., served and taken as side dishes, and is normally packaged in a small
package. "Donburi" is an amount of rice with a topping of, for example,
boiled meat, tempura, sea food, etc., and is normally packaged in a
medium-sized deep bowl-like package. "Bentou" is an assortment of rice and
some side dishes packaged together, like a so-called TV dinner, and the
package is normally large and flat. The curve A3 in FIG. 5 is the result
of heating a souzai package, the curve B2 is the result of heating a
donburi package, and the curve C1 is the result of heating a bentou
package.
The time length t0 reflects the degree of rise in the detected temperature
in the initial phase of heating. Accordingly, it can be said that the
shorter the time length t0 is, the more rapidly the detected temperature
rises in the initial phase of heating. The rise in the detected
temperature in the initial phase of heating greatly depends on the heat
capacity of the food 5d since the rise in the temperature of the air in
the package is mainly caused by the heat conducted from the food 5d. That
is, when the amount of the food 5d is large, the food 5d cannot be warmed
rapidly, so that the degree of rise in the temperature is small. For
example, when a packaged food consists of a small amount of food in a
small package, as in the case of souzai described above, the temperature
of the food rises so rapidly that the time length t0 is very short. See
the curve A3 in FIG. 5.
After the temperature at the surface of the lid 5a has risen to some
extent, the degree of rise in the detected temperature depends more on the
type of the food 5d and the size of the space between the food 5d and the
lid 5a rather than on the amount of food 5d, as explained before. For
example, when a food in a flat package, such as a bentou package as
described above, is heated, the temperature at the surface of the lid 5a
is hardly influenced by the hot steam or gas released from the food since
little or no space exists between the lid 5a and the food 5d. Therefore,
the temperature rises slowly and the temperature change .alpha. is small.
The heating control according to the sequences of FIG. 4 is explained in
detail. The first stage in FIG. 4 corresponds to steps S2-S9 in FIG. 3.
For example, the output power of the magnetron 3 is set at 1500 ›W! after
the heating is started. When the detected temperature has risen by 18
›.degree.C.!, the operation proceeds to the second stage. When the initial
temperature of the object 5 is high, the maximum temperature of 50
›.degree.C.! may be reached before the detected temperature rises by 18
›.degree.C.!. In this case, the time elapsed until then (exactly speaking,
the elapsed time minus 5 ›sec!) is adopted as the time length t0 before
proceeding to the second stage.
The second stage corresponds to step S10 and the subsequent steps. The
heating power is still maintained at 1500 ›W!. When 5 ›sec! has elapsed
since entry into the second stage, one of the heating modes is selected
according to the time length t0 and the temperature change .alpha.. When
the detected temperature attains the maximum temperature, the operation
proceeds to the third stage. If the maximum heating time elapses before
the maximum temperature is attained, the operation also proceeds to the
third stage at that time. In case that, referring back to step S10, the
maximum temperature of 60 ›.degree.C.! is attained while the temperature
change .alpha. is being measured (i.e. while the operation is suspended
for 5 ›sec!) in step S10, the temperature change .alpha. (which is defined
for 5 ›sec!) is calculated based on the time needed for the detected
temperature to rise from the initial temperature to the maximum
temperature 60 ›.degree.C.!. Then, one of the heating modes is selected
according to the time length t0 and the temperature change .alpha., and
the operation proceeds to the third stage.
In the third stage, the heating is continued keeping the power at 1500 ›W!.
The heating is finished when the detected temperature attains the maximum
temperature predetermined corresponding to the selected heating mode. The
heating is otherwise finished when the maximum heating time (0.7.times.t)
elapses before the detected temperature rises to the maximum temperature.
For example, referring to the curve B2 in FIG. 5, time length t0 in the
first stage is 25 ›sec! and the temperature change .alpha. after the start
of the second stage is 9 ›.degree.C.!. Therefore, the heating mode B2 is
selected when 5 ›sec! elapses since the start of the second stage (i.e. at
the point x in FIG. 5). According to the heating mode B2, the maximum
heating time in the second stage is set at 0.75 ›min! (45 ›sec!) and the
maximum temperature is set at 60 ›.degree.C.!. At the point y in FIG. 5,
the detected temperature attains the maximum temperature of 60
›.degree.C.! before the maximum heating time of 45 ›sec! elapses since the
start of the second stage. Thus, at the point y, the operation proceeds
from the second stage to the third stage. The length of time t from the
start of the first stage to the end of the second stage is 37 ›sec!.
Therefore, the maximum heating time in the third stage is set at
0.7.times.37=26 ›sec! and the maximum temperature is set at 65
›.degree.C.!. At the point z in FIG. 5, the detected temperature attains
the maximum temperature of 65 ›.degree.C.! before the maximum heating time
of 26 ›sec! elapses. Hence, at the point z, the magnetron 3 is stopped and
the heating is completed.
Normally, the maximum temperature is attained in each stage before the
maximum heating time elapses as described above. In other words, the
maximum heating time is provided for unusual cases as follows. When the
object 5 is not placed in the proper position, the infrared sensor 8
detects the temperature of the turntable 6 instead of the temperature of
the lid 5a. The maximum heating time prevents overheating of the food in
such a case, and assures safety.
Also, the hygrosensor 10 may be utilized for safety. For example, such a
situation should be considered that the object 5 is placed out of the
detectable scope of the infrared sensor 8. In this case, an abnormal
reference level for the detection signal of the hygrosensor 10 is
predetermined in each stage, and when the detection signal exceeds the
abnormal reference level, the operation proceeds to the next stage as when
the detected temperature has attained the maximum temperature (or the
operation is stopped).
In the microwave oven of the above embodiment, a plurality of input keys
may be provided to the operation unit 23 and a plurality of heating
sequences may be prepared corresponding to respective input keys. For
example, the input keys may be provided corresponding to various types of
packaged food such as "souzai", "donburi", "bentou", "onigiri" (rice ball)
and "bread". The input keys may be otherwise provided corresponding to the
preserving states of the food, or the temperature of food before a heating
is started, such as "freezed", "chilled" and "preserved at normal
temperature". By the microwave oven as described above, various types of
packaged foods can be heated more appropriately.
In the above embodiment, the time required for the temperature to rise by a
predetermined amount is used as the parameter representing the degree of
rise in the temperature in the initial phase of the heating and the change
in the temperature per unit time is used as the parameter representing the
degree of rise in the temperature in the following phase of the heating.
Of course the degree of rise in the temperature in the initial phase may
be represented by the change in the temperature per unit time, and the
degree of rise in the temperature in the following heating phase may be
represented by the time required for the temperature to rise by a
predetermined amount. In the initial phase, however, the inclination of
the temperature curve changes much as the time elapses, as shown in FIG.
5. Therefore, the degree of rise in the temperature in the initial phase
of the heating can be detected more precisely and easily by the method of
the above embodiment.
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